Many low voltage electronic circuits have a need to drive high voltage switching transistors through an isolating barrier. This need stems from the fact that a large voltage differential, typically in the order of 100 volts, exists between the low voltage control logic and the high voltage power switch. Therefore, for safety and other reasons, the control logic and high voltage switch must be electrically isolated.
Several circuits addressing these problems have been previously proposed. For example, circuits utilizing a number of low voltage power supplies coupled to the power switches, high-speed opto-couplers, or other extensive electrical isolation circuits have been proposed, although all have met with limited success due to problems associated with their excessive size and cost, or low reliability due to the large number of components.
One known circuit is described in U.S. Pat. No. 5,206,540 issued to de Sa e Silva et al (hereinafter referred to by '540). The '540 patent discloses a transformer isolated circuit for driving a semiconductor power switch that includes a transformer which is driven with a high frequency Pulse Width Modulated (PWM) signal in such a way that the constantly present PWM carrier supplies the power needed for high gate current pulses, while different voltage levels determine the "ON" or "OFF" state of the power device under control. Although transformer saturation is detected and prevented via the primary current, this circuit fails to provide feedback through the transformer itself to detect short circuits, desaturations, or collector/drain voltage.
Thus, a need exists for a low cost and reliable high power inverter that provides for reverse fault communications.